L is for Living Relatives

Dinosaurs could see in color. The EPB-living relative theory says so. Picture by Sergey Krasovskiy.

EPB. Remember those letters when you think of dinosaurs. They’re hard words–extant phylogenetic bracket–which I will define shortly. But they are like a magic wand for paleontologists and paleobiologists. EPB lets scientists looking at fossil bones, those 100-million-year old rocks, tell what kind of muscles they had, whether their blood vessels were strong, and whether they could see in color. Scientists can tell all sorts of things about the soft tissues inside those bones because they can compare them to the closest Living Relative. (I was going to include this under letter E, but I had to talk about extinction, so I’m slipping it in here under L. By inference, which is how EPB works.)

EPB: Big Words, Brilliant Idea

Let’s break this acronym down. Extant is the opposite of extinct, so that refers to something living, in particular a species or group of animals (remember C for Clade). Phylogenetic is a mouthful. Phylo means group and genetic refers to a group. Bracket also means group.

extant (living) + phylogenetic (group evolutionary tree) + bracket (group) = EPB

If you ever had biology way back when, you might have bemoaned having to learn the phrase “ontogeny recapitulates phylogeny.” Maybe you remember it as wah-wah-wah wah-wah-wah wah-wah. This was an old idea that when embryos develop, they form stages similar to the evolution of animals: looking like fish, then tadpoles, then pigs, then human babies, for example. It was one of those Enlightenment ideas when (straight white wealthy male European) scientists thought they could figure out everything and classify it all into a controllable system. Which they can’t. The point is that you can distinguish ontogeny, the growth of a single individual, from phylogeny, the growth and adaptation of a species along the evolutionary tree. Think of phylogeny as the tree.

Drawing EPB conclusions about Stenotosaurus, from wikipedia.

This EPB idea refers to the traits now shared by an existing group along that branching tree of development. In 1995, Lawrence Witmer introduced this idea that you could determine all sorts of things about the soft-tissue biology of long-dead creatures by looking at their existing descendants. The EPB lets you draw conclusions by inference in ways that you can’t do by direct testing since the physical material doesn’t exist. You can’t look at the muscles, eye cells, or the hearts of dinosaurs, but you can draw conclusions based on other like animals.

Chubby Cheeks

Take the example of the cheeks. One topic long-debated by those who study the giant plant-eaters is how their cheek muscles were designed. Of course, they had big jaw muscles, and we know they ate plants because of the shape of their teeth. That was the whole point of Mantell’s naming his found bones Iguanodon –they had iguana-like teeth. Iguanadons ate plants. In contrast, pointy teeth are for ripping meat. Flat or tipped teeth are for plants. Paleontologists have long known that some dinosaurs didn’t even chew the plants because the teeth aren’t able to grind, yet aren’t sharp enough to bite anything stronger than a leaf. Those tale brachiosauruses just pulled and swallowed.

Triceratops teeth, lots for chewing plants. Photo from fossilera.

Teeth tell you what they ate, but they can’t tell you entirely how they ate it. Flat teeth means the leaves were ground up, but were they ground side-to-side or up-and-down? Triceratops, as it turns out, had five different layers in their teeth, one of the most sophisticated tooth structures every found. Plus, huge heads, so chewing was a complicated endeavor. Where did their muscles go?

According to Ali Nabavizadeh, the old theory of up-and-down muscles in the cheeks was wrong. The idea limited the way their mouths could move, but the cheek muscles were thought to be constructed this way because that’s the way mammals chew. Yet mammals aren’t that similar to dinosaurs. Living descendants of dinosaurs, namely reptiles, have different cheek structures.

Nabavizadeh argues that it makes more sense to look at muscles across the cheeks as additional extensions of the jaw muscles. These are more similar to reptiles as well as similar to known descendants on the tree. It would suggest that the jaw went in a backwards motion, which fit the sophisticated teeth better than a model like current mammals.

New cheek muscle model by Nabavizadeh.

The Old Blood Question

One of the biggest, long-standing dinosaur debates involves whether dinosaurs were cold-blooded or warm-blooded. Modern reptiles are cold-blooded; they get their energy from external heat sources. Modern birds and mammals are warm-blooded; they produce energy internally from what they eat and moving around. Dinosaurs are reptiles, so it was thought they were slow and sluggish, requiring sunlight as energy. Being cold-blooded would restrict where they lived, how they moved, and what they could do. Being warm-blooded would allow for more variation in behavior, habitat, and diet–everything!

Could looking at living relatives solve this question? Scientists tried to do it by thinking about dinosaur hearts. Dinosaurs skeletons could suggest how big a heart might be. One of the giant sauropods might have had hearts that weighed 600 pounds. But what about its design? Living relatives of dinosaurs, both crocodiles and birds, both had four-chambered hearts. Scientists are fairly certain that dinosaurs, too, had four-chambered hearts. Mammals also have four-chambered hearts. You need sophisticated hearts when you are pumping a lot of blood all over a body the size of an airplane.

Clip from”Nature” show, Dinosaur hearts.

But mammals and birds are warm-blooded, while crocodiles are cold-blooded. All of them have four-chambered hearts. However, the heart construction alone can’t tell you whether the dinosaurs were warm or cold-blooded, since their living ancestors–birds and crocodiles–had different metabolic systems. EPB here tells you that you don’t know.

Color Vision

Using the living ancestor model for vision can show further how far you can and can’t go. Looking at a dinosaur skeleton can tell you a lot about vision. Some dinosaurs had eyes that faced forward and others had great depth perception. But how could you tell about color perception, and what would you know if you did?

Similar animals descending along the phylogenetic tree have color vision. Both birds and crocodiles see in color. That allows for a pretty strong conclusion that dinosaurs could see in color. However, crocodiles can’t distinguish ultraviolet (UV), while birds can. Because of that difference, you can’t necessarily know whether UV was a trait dinosaurs had or whether birds developed it independently along the way. You can’t simply look at one descendant of a dinosaur, pick a trait, and decide all the dinosaurs had that trait. You have to look at multiple animals and go back to pick something as a common ancestor of all.

However, there was another study by scientists who identified an actual gene–CYP2J19–that lets birds see a sophisticated range of colors in red. These researchers then found the same gene in turtles. Ah-ha! They concluded that this gene very likely existed before turtles and birds split in the evolutionary tree, which means dinosaurs probably had the gene. What this “red gene” allows birds and turtles to do is distinguish a lot of different variations in red, which means they can also display a huge variation of red colors themselves. Darwin would say there might be plenty of reasons for that adaptation.

The excellent red spectrum vision provided by the CYP2J19 gene would help female birds and turtles pick the brightest red males…

Hanlu Twyman, Cambridge researcher

If they can see sophisticated shades of red, then they can display sophisticated shades of red, too. Which is why we don’t need to think of all dinosaurs as green.

Dino colors, graphic from NPR.

And why maybe we can imagine them doing what those birds are doing right now, in the spring, that little dance of allure?

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